Apparatus and method for scalable and dynamic traffic engineering in a data communication network

ABSTRACT

A global path identifier is assigned to each explicit route through a data communication network. The global path identifier is inserted into each packet as the packet enters a network and is used in selecting the next hop. When encountering a new selected path, an ingress router sends an explicit object to downstream nodes of the path to set up explicit routes by caching the next hop in an Explicit Forwarding Information Base (“EFIB”) table. Ingress routers maintain an Explicit Route Table (“ERT”) that tracks the global path identifier associated with each flow through the network. Multiple flows using the same path can be implemented by sharing the same global path identifier. In case of sudden network load changes, rerouting can be performed by changing the global path identifier associated with those flows that need to be rerouted and by then transmitting a new path object to downstream nodes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to data communication networks. Moreparticularly, the present invention relates to a scalable apparatus andmethod for dynamic selection of explicit routes and dynamic rerouting indata communication networks and internetworks such as the Internet.

2. Background

As is known to those of ordinary ski in the art, a network is acommunication system that allows users to access resources on othercomputers and exchange messages with other users. A network is typicallya data communication system that links two or more computers andperipheral devices. It allows users to share resources on their ownsystems with other network users and to access information on centrallylocated systems or systems that are located at remote offices. It mayprovide connections to the Internet or the networks of otherorganizations. The network typically includes a cable that attaches tonetwork interface cards (“NICs”) in each of the devices within thenetwork. Users may interact with network-enabled software applicationsto make a network request (such as to get a file or print on a networkprinter). The application may also communicate with the networksoftware, which may then interact with the network hardware to transmitinformation to other devices attached to the network.

FIG. 1 is a block diagram illustrating an exemplary network 100connecting a user 110 and a particular web page 120. FIG. 1 is anexample that may be consistent with any type of network known to thoseof ordinary skill in the art, including a Local Area Network (“LAN”), aWide Area Network (“WAN”), or a combination of networks, such as theInternet.

When a user 110 connects to a particular destination, such as arequested web page 120, the connection from the user 110 to the web page120 is typically routed through several internetworking devices such asrouters 130-A-130-I. Routers are typically used to connect similar andheterogeneous network segments into internetworks. For example, two LANsmay be connected across a dialup, integrated services digital network(“ISDN”), or across a leased line via routers. Routers may also be foundthroughout internetwork known as the Internet End users may connect to alocal Internet service provider (“ISP”) (not shown).

As shown in FIG. 1, multiple routes are possible to transmit informationbetween user 110 and web page 120. Networks are designed such thatrouters attempt to select the best route between computers such as thecomputer where user 110 is located and the computer where web page 120is stored. For example, based on a number of factors known to those ofordinary skill in the art, the route defined by following routers 130-A,130-B, 130-C, and 130-D may be selected. However, the use of differentrouting algorithms may result in the selection of the route defined byrouters 130-A, 130-E, 130-F, and 130-G, or possibly even the routedefined by routers 130-A, 130-B, 130-H, 130-I, 130-F, and 130-G. Adetailed discussion of the aspects of routing algorithms that determinethe optimal path between two nodes on a network is not necessary for thepurposes of the present invention, and such a discussion is not providedhere so as not to overcomplicate the present disclosure.

Routers such as routers 130-A-130-I typically transfer information alongdata communication networks using formatted data packets. For example,when a “source” computer system (e.g., computer 110 in FIG. 1) wishes totransmit information to a “destination” computer system (e.g., computer120 in FIG. 1), it generates a packet header in an appropriate formatwhich typically includes the address of the source and destination endsystem, and then fills the remainder of the packet with the informationto be transmitted. The complete data packet is then transmitted to therouter attached to (and responsible for) the source computer system,which forwards it toward the destination computer system. Packetstransmitted among the routers themselves (typically referred to as“control packets”) are similarly formatted and forwarded.

When a router receives a data packet, it reads the data packet'sdestination address from the data packet header, and then transmits thedata packet on the link leading most directly to the data packet'sdestination. Along the path from source to destination, a data packetmay be transmitted along several links and pass through several routers,with each router on the path reading the data packet header and thenforwarding the data packet on to the next “hop.”

To determine how data packets should be forwarded, each router istypically aware of the locations of the network's end systems (i.e.,which routers are responsible for which end systems), the nature of theconnections between the routers, and the states (e.g., operative orinoperative) of the links forming those connections. Using thisinformation, each router can compute effective routes through thenetwork and avoid, for example, faulty links or routers. A procedure forperforming these tasks is generally known as a “routing algorithm.”

FIG. 2 is a block diagram of a sample router 130 suitable forimplementing an embodiment of the present invention. The router 130 isshown to include a master control processing unit (“CPU”) 210, low andmedium speed interfaces 220, and high speed interfaces 230. The CPU 210may be responsible for performing such router tasks as routing tablecomputations and network management. It may include one or moremicroprocessor integrated circuits selected from complex instruction setcomputer (“CISC”) integrated circuits, reduced instruction set computer(“RISC”) integrated circuits, or other commercially available processorintegrated circuits. Non-volatile RAM and/or ROM may also form a part ofCPU 210. Those of ordinary skill in the art will recognize that thereare many alternative ways in which such memory can be coupled to thesystem.

The interfaces 220 and 230 are typically provided as interface cards.Generally, they control the transmission and reception of data packetsover the network, and sometimes support other peripherals used withrouter 130. Examples of interfaces that may be included in the low andmedium speed interfaces 220 are a multiport communications interface222, a serial communications interface 224, and a token ring interface226. Examples of interfaces that may be included in the high speedinterfaces 230 include a fiber distributed data interface (“FDDI”) 232and a multiport Ethernet interface 234. Each of these interfaces(low/medium and high speed) may include (1) a plurality of portsappropriate for communication with the appropriate media, and (2) anindependent processor, and in some instances (3) volatile RAM. Theindependent processors may control such communication intensive tasks aspacket switching and filtering, and media control and management. Byproviding separate processors for the communication intensive tasks,this architecture permits the master CPU 210 to efficiently performrouting computations, network diagnostics, security functions, and othersimilar functions.

The low and medium speed interfaces are shown to be coupled to themaster CPU 210 through a data, control, and address bus 240. High speedinterfaces 230 are shown to be connected to the bus 240 through a fastdata, control, and address bus 250, which is in turn connected to a buscontroller 260. The bus controller functions are typically provided byan independent processor.

Although the system shown in FIG. 2 is an example of a router suitablefor implementing an embodiment of the present invention, it is by nomeans the only router architecture on which the present invention can beimplemented. For example, an architecture having a single processor thathandles communications as well as routing computations would also beacceptable. Further, other types of interfaces and media known to thoseof ordinary skill in the art could also be used with the router.

At a higher level of abstraction, FIG. 3 is a block diagram illustratinga model of a typical router system that is applicable in the context ofthe present invention. As shown in FIG. 3, a networking device such as arouter 130 may be modeled as a device having a plurality of inputinterfaces 310 a-310 n, each having a corresponding input interfacequeue 320 a-320 n. Each input interface 310 receives a stream 330 a-330n of data packets 340 a-340 z, with each data packet 340 typicallyarriving at a variable rate and typically having a variable length(usually measured in bytes). In addition to the data “payload” in eachpacket, each packet contains header information, which typicallyincludes a source address and a destination address. Currently, thedominant protocol for transmitting such data packets is the InternetProtocol (“IP”). However, as will be described more fully in subsequentportions of this document, embodiments of the present invention can beimplemented using any routing protocol known to those of ordinary skillin the art.

As each new data packet 340 arrives on an interface 310 k, it is writteninto a corresponding input interface queue 320 k, waiting for its turnto be processed. Scheduling logic 350 determines the order in whichinput interfaces 310 a-310 n should be “polled” to find out how manydata packets (or equivalently, how many bytes of data) have arrived on agiven interface 310 k since the last time that interface 310 k waspolled. Scheduling logic 350 also determines the amount of data thatshould be processed from a given interface 310 k during each “pollinground.”

Regardless of the specific form of scheduling logic 350 used, whenscheduling logic 350 determines that a particular data packet 340 ishould be processed from a particular input interface queue 320 k,scheduling logic 350 transfers the data packet 340 i to subsequentportions of the networking device (shown as dashed block 355) forfurther processing. Eventually, data packet 340 i is written into one ofa plurality of output queues 360 a-360 q, at the output of which thedata packet 340 i is finally transmitted from the networking device thecorresponding output interface 370 a-370 q. Fundamentally, then, thepacket forwarding component of a router performs the function ofexamining the source and destination address of each data packet andidentifying one from among a plurality of output interfaces 370 a-370 qon which to transmit each data packet.

FIG. 4 is a flow chart illustrating a more detailed view of the packetforwarding operations performed on a typical router. As shown in FIG. 4,at step 405, a data frame is received, and in step 410, it is strippedof all its data link layer frame information. The exposed data fieldthen contains the actual network layer data packet. Next, at step 415,the network layer protocol is identified. As mentioned earlier, thereare several network layer protocols that may be routed in a datacommunication network, with the Internet Protocol (“IP”) being dominantin current wide area networks such as the Internet.

Still referring to FIG. 4, after identifying the network protocol instep 415, the router determines whether the protocol is one that can berouted in step 420. If the protocol is not one that can be routed, thenat step 425, the link layer header is added back to the received dataframe, and at step 430 the frame is simply transmitted using link layertechniques known to those of ordinary skill in the art, such asswitching or bridging.

If the packet protocol is determined to one that can be routed in step420 (e.g., if it is an IP packet), then at step 435, the router firstperforms housekeeping functions known to those of ordinary skill in theart, and then the router “looks up” the destination IP address in itsrouting table to identify the appropriate router output interface (alsocalled a “port”) on which to transmit the received packet. At step 440,the router determines whether the destination port is directly attachedto the router. If so (i.e., if the destination port is another port onthe router), then at step 445, the link layer header is added back tothe packet with the original link layer destination address, and then atstep 450 the reassembled link layer frame is transmitted through theport identified at step 435.

Otherwise, if at step 440 the router determines that the destinationport is not directly attached to the router (indicating that anotherrouter hop needs to occur), the Media Access Control (“MAC”) address ofthe next hop may be added to the packet, and a new link layer headerwith this MAC destination address extracted from the routing table isadded to the frame at step 455, and then at step 460 the frame istransmitted through the port identified at step 435.

With the integration of voice, video, and data traffic, the aggregatebandwidth requirement of applications is getting higher. More networkinfrastructures are being built with high-end routers and switches witha large number of interfaces. As a result, network connectivity isgetting richer than ever before. Multiple selections of routes for anygiven source and destination pair are often available. However, as isknown to those of ordinary skill in the art, existing routing protocols,such as the Open Shortest Path First (“OSPF”) protocol, the RoutingInformation Protocol (“RIP”), the Enhanced Interior Gateway RoutingProtocol, and the Border Gateway Protocol (“BGP”) are essentiallydestination-based routing protocols, which means that all packets withthe same destination are typically forwarded along a minimum-hop path totheir destination.

With destination-based routing protocols, routes are typicallycalculated automatically at regular intervals by software in routingdevices. To enable this type of dynamic routing, routing devices containrouting tables, which essentially consist of destination address/nexthop pairs. As an example, an entry in a routing table (i.e., a routingtable as used in step 435 of FIG. 4) may be interpreted as follows: toget to network IP address 123.045.0.0, send the packet out Ethernetinterface ‘0’ (e.g., “E0”). Thus, IP datagrams, or packets, travelthrough internetworks one hop at a time. The entire route is not knownat the onset of the journey, however. Instead, at each hop, the nextdestination is calculated by matching the destination address within thedatagram with an entry in the current node's routing table. Each node'sinvolvement in the routing process is limited to forwarding packetsbased on internal information. The nodes typically do not monitorwhether the packets get to their final destination, nor does the IPprotocol have the capability to report errors back to the source whenrouting anomalies occur.

Although networks are designed to match expected traffic load, theactual traffic load in data communication networks changes dynamicallydepending on the time of day, and from day to day as well. A specialevent (e.g., the final game of the soccer World Cup) can dramaticallychange the network traffic pattern. In addition, network devices canbecome disabled or may be added to a network, and these events can alsochange the network traffic pattern.

As is known to those of ordinary skill in the art, destination-basedminimum-hop routing algorithms (such as OSPF, RIP, and BGP) typicallyselect the “shortest path” to a destination based on a metric such ashop count. These protocols therefore have limited capacity to balancethe network load. With these protocols, when the traffic load isconcentrated, some links are heavily loaded, while others sit idle.Thus, when using these routing algorithms, a network cannot dynamicallyadjust its forwarding paths to avoid congested links.

Traffic engineering is an important network service that achievesnetwork resource efficiency by directing certain traffic flows to travelthrough explicitly defined routes that are different from the defaultpaths determined by the routing protocols (e.g., OSPF, EIGRP, or BGP).As is known to those of ordinary skill in the art, the key to successfultraffic engineering is the ability to have a set of network mechanismsthat supports both explicit routes and dynamic rerouting. Dynamicrerouting refers to the ability to periodically recalculate routes in adata communication network depending on network load or other factors.

Fundamentally, traffic engineering is a function of routing. However,existing destination-based routing protocols, such as the protocolsalready mentioned, make it very difficult—if not impossible—to supportexplicit routes.

One current approach proposed by the Internet Engineering Task Force(“IETF”) toward supporting explicit routes in traffic engineeringapplications is to use the multiple label switching (“MPLS”) techniqueand the Resource Reservation Protocol (“RSVP”) to set up explicitroutes. MPLS and RSVP are not discussed in detail in this document, soas not to overcomplicate the present discussion. For the purposes of thepresent discussion, however, suffice it to say that in both the MPLS andRSVP techniques, a technique known as “tag switching” can be used,wherein explicit routes are cached in Tag Information Base (“TIB”)entries. In general, tag switching techniques work as follows: at theedge of a tag-switched network, a tag is applied to each packet A taghas only local significance. When a packet is received by a tag switch(e.g., a router or ATM switch with tag switching software), the switchperforms a table look-up in the TIB. Each entry in the TIB consists ofan incoming tag and one or more subentries of the form: outgoing tag,outgoing interface, outgoing link layer information. The tag switchreplaces the tag in the packet with the outgoing tag and replaces thelink layer information. The packet is then sent out on the givenoutgoing interface. It should be noted that the TIB is built at the sametime that the outing tables are populated, not when the tag is neededfor the first time, which allows flows to be switched starting with thefirst packet.

According to the tag switching techniques that can be implemented withMPLS and RSVP, at each node in a route, an incoming packet is forwardedalong the explicit route to the next hop stored in the TIB whose indextag matches the tag in the packet header. However, the main problem withthis approach is its inherent complexity. Both RSVP and MPLS are verycomplicated and computationally intensive protocols, and they must beextended in order to support explicit routes. Moreover, to supporttraffic engineering, a network must support both the RSVP and MPLSprotocols, and this can significantly limit the scope of trafficengineering available. This is because new service models, such asdifferentiated services, do not require the hop-by-hop signalingprovided by RSVP. Moreover, this approach to supporting dynamic trafficengineering (i.e., making routing decision on a “per-flow” basis), isinflexible and expensive. Moreover, as is known to those of ordinaryskill in the art, the current approach is not scalable because the sizeof the TIB that is required for a network with a given number of nodesgrows exponentially as a function of the number of nodes.

The present invention provides a technique for scalable and dynamicrerouting that significantly reduces the complexity of trafficengineering as currently proposed in the IETF. As described herein,according to aspects of the present invention, a global path identifieris assigned to each explicit route in a data communication network. Inone embodiment, this global path identifier is inserted in the optionalfield of an IP packet header, and is used in selecting the next hop by arouter's forwarding engine. As another example, the global pathidentifier can be inserted as a label in MPLS systems. Explicit routescan be selected either by a policy server or by ingress routers. Whenencountering a new selected path, an ingress router sends an explicitobject to downstream nodes of the path to set up explicit routes bycaching the next hop in an Explicit Forwarding Information Base (“EFIB”)table in each router along the route. Two explicit routes that merge ata network node will share the same entry in the EFIB tables in alldownstream nodes. Ingress routers maintain an Explicit Route Table(“ERT”) table that tracks the global path identifier associated witheach flow through the data communication network. Multiple flows usingthe same path can be implemented by sharing the same global pathidentifier in the table. In case of sudden network load changes,rerouting can be performed by changing the global path identifierassociated with those flows that need to be rerouted, and by thentransmitting a new path object to downstream nodes.

Compared with the existing approach, the technique according to aspectsof the present invention is routing protocol independent, scalable, anddynamic, and it can support both class-based and flow-based explicitroutes. These and other features and advantages of the present inventionwill be presented in more detail in the following specification of theinvention and in the associated figures.

SUMMARY OF THE INVENTION

A global path identifier is assigned to each explicit route through adata communication network. This global path identifier can be obtainedby performing the bitwise Exclusive OR function of all of the uniqueidentifiers, such as IP addresses, of all the nodes along each explicitroute. The global path identifier is inserted into each packet as thepacket enters a network, and is used in selecting the next hop by arouter's forwarding engine. Explicit routes can be selected either by apolicy server or by ingress routers. When encountering a new selectedpath, an ingress router sends an explicit object to downstream nodes ofthe path to set up explicit routes by caching the next hop in anExplicit Forwarding Information Base (“EFIB”) table in each router alongthe route. Two explicit routes that merge at a network node will sharethe same entry in the EFIB tables in all downstream nodes. Ingressrouters maintain an Explicit Route Table (“ERT”) table that tracks theglobal path identifier associated with each flow through the datacommunication network. Multiple flows using the same path can beimplemented by sharing the same global path identifier in the table. Incase of sudden network load changes, rerouting can be performed bychanging the global path identifier associated with those flows thatneed to be rerouted, and by then transmitting a new path object todownstream nodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate an embodiment of the inventionand, together with the present description, serve to explain theprinciples of the invention.

In the drawings:

FIG. 1 is a block diagram illustrating an exemplary network connectionbetween a user and a web page.

FIG. 2 is a block diagram of an exemplary router suitable forimplementing an embodiment of the present invention.

FIG. 3 is a block diagram illustrating a packet processing model in atypical router having multiple input interfaces and multiple outputinterfaces suitable for implementing an embodiment of the presentinvention.

FIG. 4 is a flow chart illustrating a typical packet forwardingoperation in a router suitable for implementing an embodiment of thepresent invention.

FIG. 5 is a block diagram of a typical computer internetwork including acollection of domains having one or more local networks.

FIG. 6 is a block diagram of a computer internetwork including acollection of domains connected to a backbone via one or more routers.

FIG. 7 is a block diagram illustrating an exemplary network in which anembodiment of the present invention has been implemented.

FIG. 8 is a block diagram illustrating the structure of an exemplaryExplicit Route Table (“ERT”) according to aspects of the presentinvention.

FIG. 9 is block diagram illustrating the structure of an exemplaryExplicit Forwarding Information Base (“EFIB”) according to aspects ofthe present invention.

FIG. 10 is a flow chart illustrating the process of setting up an ERTaccording to an embodiment of the present invention.

FIG. 11 is a flow chart illustrating the process of setting up an EFIBaccording to an embodiment of the present invention.

FIG. 12 is a flow chart illustrating the packet forwarding process at aningress router according to an embodiment of the present invention.

FIG. 13 is a flow chart illustrating the packet forwarding process at acore router according to an embodiment of the present invention.

DETAILED DESCRIPTION

Those of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and not in anyway limiting. Other embodiments of the invention will readily suggestthemselves to such skilled persons, having the benefit of thisdisclosure. Reference will now be made in detail to an implementation ofthe present invention as illustrated in the accompanying drawings. Thesame reference numbers will be used throughout the drawings and thefollowing description to refer to the same or like parts.

As is known to those of ordinary skill in the art, internetworking isthe process of establishing and maintaining communications between andtransferring data among multiple local networks in a distributed networksystem. FIG. 5 depicts an exemplary embodiment of an internetwork system500, comprising a plurality of routing domains 504-508 coupled to abackbone network 502. In the hierarchically-arranged distributedinternetwork system 500, backbone 502 is the top-level, or central,connection path shared by the nodes and networks connected to it. Thebackbone manages the bulk of the traffic between communicating nodes toprovide end-to-end service between one user (i.e., a source node) andanother user (i.e., a destination node). Each routing domain in theinternetwork is a collection of one or more local networks that areattached to the backbone through one or more routers. Routing domainsare also referred to as “customer networks,” or “autonomous systems.”

The routers within a routing domain manage communications among localnetworks within their domain and communicate with each other using anintra-domain routing protocol. As is known to those of ordinary skill inthe art, examples of such protocols are the IP Routing InformationProtocol (“RIP”) and the International Standards Organization (“ISO”)Integrated Intermediate System-to-Intermediate System (“IS-IS”) routingprotocol.

Domains 504-508 are connected to the backbone 502 through router nodes510, 512 and 514, respectively. The routing protocols implemented insuch routers are referred to as inter-domain routing protocols. Anexample of an inter-domain routing protocol is the Inter-domain RoutingProtocol (“IDRP”), wherein in IP, the Exterior Gateway Protocol (“EGP”)and the Border Gateway Protocol (“BGP”) routing protocols are known tothose of ordinary skill in the art. Although not shown in FIG. 5, morethan one inter-domain router may be used to connect a domain to thebackbone. Such a configuration is often encouraged for redundancypurposes.

Thus, the hierarchically-arranged distributed internetwork system 500contains levels of subnetworks, each having an associated routing level.The lower routing level includes intra-domain routers 530-534 whichmanage individual links and nodes within their respective domains. Thehigher routing level includes inter-domain routers 510-514 which manageall the lower-level domains without addressing details internal to lowerrouting levels. Communications among these routers typically comprisesan exchange (i.e., “advertising”) of routing information. This exchangeoccurs between routers at the same routing level (referred to as “peer”routers) as well as between routers at different routing levels.

In order to reduce design complexity, most networks are organized as aseries of hardware and software levels, or “layers,” within each node.These layers interact to format data for transfer between, for example,a source node and a destination node communicating over the network.Specifically, predetermined services are performed on the data as itpasses through each layer and the layers communicate with each other bymeans of predefined protocols. This layered design permits each layer tooffer selected services to other layers using a standardized interfacethat shields those layers from the actual implementation details of theservices.

As was mentioned earlier, the destination address typically stored inthe header of a data packet can be used by the network layer protocolsto determine the route for the packet. As is known to those of ordinaryskill in the art when a packet is received at the network layer, thatlayer examines the network layer header of the packet, determines thenext hop that the packet should take based upon the destination address,appends a new network layer header onto the packet as necessary, andpasses the modified packet to the data link layer responsible for theoutgoing link associated with the next hop.

Since network layer addresses are hierarchical in nature, network layerprotocols that perform routing functions at the same routing level of aninternetwork make next hop determination based upon the same portion ofthe destination address referred to as the destination address prefix.The routing information exchanged by peer routers typically includesthis destination address prefix.

In conventional routers, these destination address prefixes are storedin a forwarding information database (“FIB”). To determine the next hop,network layer protocols typically implement alongest-matching-address-prefix (“longest match”) algorithm thatsearches the forwarding database for an entry corresponding to thedestination address located in the network layer header. The datastructure of the forwarding database comprises a large number of“branches,” each representing a string of the hierarchical destinationaddress fields. The branches of the database have different addressfield values and/or terminate with a different number of address fields.The longest match algorithm must traverse numerous branches during thisaddress matching process to determine the next hop. Due to thehierarchical characteristics of destination addresses, there may beseveral destination address prefixes in the forwarding database thatmatch at least a portion of a particular destination address. As aresult, the longest address in the forwarding database must be found.

As is known to those of ordinary skill in the art, the time required tosearch the forwarding database is proportional to the average length ofthe destination address in the database. Destination addresses oftenhave a significant number of bits, particularly in large,hierarchically-arranged internetworks. Due to the length of thedestination addresses, as well as the manner in which the longest matchalgorithm navigates the forwarding database, the address matchingfunctions contribute significantly to the overhead associated with thepacket forwarding process performed by network layer protocols, therebyadversely affecting the router's efficiency.

FIG. 6 is a block diagram of an internetwork system 600 comprising acollection of domains 602-604 containing a number of local computernetworks 616-622, each having one or more nodes, such as workstationsand servers. The local networks are connected by intermediate nodes,such as intra-domain routers 624-628 and inter-domain routers 606-614implementing the techniques of the present invention. Each intra-domainrouter 624-628 and each inter-domain router 606-614 may have a hardwarearchitecture similar to that shown in FIG. 2 and described earlier.

Although all routers perform similar tasks, there are some importanthardware and software differences between the functions performed byintra-domain routers and inter-domain routers. Inter-domain routers,also known as “edge routers,” or “ingress routers,” are primarilyconcerned with network topology and maintenance, while intra-domainrouters, also known as “core routers” are primarily devoted to packetforwarding.

As shown in FIG. 6, intra-domain routers 624-626 manage communicationsamong local networks 616, 618 within domain 602 and communicate witheach other using an intra-domain routing protocol, such as RIP or IS-IS.Inter-domain routers 606-614 connect domains 602 and 604 to the backbone640 and communicate with each other using an inter-domain routingprotocol, such as IDRP, EGP or BGP. Peer routers exchange (i.e.,advertise) routing information with each other and maintain the routinginformation in a forwarding information database (“FIB”) discussedbelow.

The routers include connectionless network layer protocols such as theconnectionless network layer protocol (“CLNP”), the Internet Protocol(“IP”) network layer protocol and the Internetwork Packet Exchange(“IPX”) protocol. Connectionless network layer protocols exchangerouting information with each other using neighbor greeting protocolssuch as the Address Resolution Protocol (“ARP”). This information isstored in the forwarding information database and used by the networklayer protocols to determine paths through the internetwork 600.

Primarily, when an inter-domain router receives a packet from a previoushop inter- or intra-domain router, it determines the next hop inter- orintra-domain router to which to forward the packet using a packetforwarding system 636, of which many are known to those of ordinaryskill in the art. Similarly, intra-domain routers forward packetsbetween a previous hop inter-domain router and the end nodes, makingnext hop determination using the packet forwarding system 636. Forexample, as shown in FIG. 6, intra-domain router 626 forwards packetsoriginating at source node 642 to inter-domain router 608, which thenforwards the packet through backbone 640 to, for example, inter-domainrouter 612, which then forwards the packet to intra-domain router 628,which then finally transfers the packet to destination end node 646. Aswill be described in more detail below, the present invention may beimplemented in both intra-domain and inter-domain routers or othernetwork devices.

The present invention employs a novel approach to support dynamicselection of explicit routes and dynamic rerouting in the Internet. Theproposed technique is simple and scalable. It can support bothflow-based and class-based dynamic traffic engineering. The technique isindependent of existing routing protocols, but uses the route selectedby existing routing protocols as default routes. Hence, it can be usedin both intra-domain (e.g., OSPF, RIP, and EIGRP) and inter-domain(e.g., BGP) routing.

Most packet-switched networks employ some form of shortest-path routing.The objective of shortest-path routing is to determine a least-cost pathbetween a source and a destination. The cost can be hop count, delay,bandwidth, queue size, or combinations of these “metrics.” Shortest-pathrouting algorithms are divided into two classes: distance vector andlink state algorithms. As is known to those of ordinary skill in theart, distance-vector algorithms are derived from the Bellman-Fordshortest-path algorithm, and base path selection on local estimates ofpath costs. These algorithms are usually implemented in a distributedasynchronous fashion. On the other hand, link-state algorithms arederived from Dijkstra's shortest-path algorithm, and are usuallyimplemented in a replicated fashion (i.e., each node performs anindependent route computation) and construct paths based on globalestimates of individual link costs.

As packet-switched networks grow to accommodates an increasing userpopulation, the amount of routing information that must be distributed,stored, and manipulated in these networks also grows. Research onrouting in large packet-switched networks has focused on ways to reducethe quantity of routing information, without sacrificing the quality ofa selected path. The majority of proposed and deployed solutions usealgorithms for hierarchical clustering of topology information,abstraction of routing information relating to these clusters, andpacket forwarding within the hierarchy.

Routing algorithms deployed in today's Internet focus on basicconnectivity and typically support only one type of datagramservice-best effort service. Moreover, current Internet routingprotocols, such as BGP, OSPF, and RIP, use “shortest path routing”(i.e., routing that is optimized for a single metric, such asadministrative weight or hop count). These routing protocols are also“opportunistic,” which means that they use the current shortest path orroute to a destination. In order to avoid loops, alternative paths withadequate but less than optimal cost cannot be used to route traffic.However, it should be noted that shortest path routing protocols doallow a router to alternate among several equal cost paths to adestination.

There are typically two methods to achieve high network throughput:conserving resources and balancing traffic load. To conserve resources,it is desirable to select paths that required the fewest possible hops,or which require the least amount of bandwidth. On the other hand, tobalance the network load, it is desirable to select the least loadedpaths. In practice, these two methods often conflict with each other,and tradeoffs must be made between load balancing and resourceconserving resources.

Packet-switched networks may be subdivided into two general categories:virtual circuit networks and datagram networks. In a virtual circuitnetwork (e.g., an Asynchronous Transfer Mode, or “ATM” network), aconnection is set up before data transmission starts and is tom downafter data transmission is completed. All packets for a connection aretransmitted in sequence from the source to the destination. In such anetwork, state information is maintained at every node along the path ofthe virtual circuit. This state information can be made available toadmission control, resource reservation, and Quality of Service (“QoS”)routing components of the network. In contrast, in a connectionlessdatagram network such as the Internet, all information 10′ needed todeliver a packet is carried in the packet header, and individualforwarding decisions are made as packets arrive in a switch or a router.No connection state information is maintained inside the network.

The concept of a connection still exists on the end-systems, but thereis no explicit connection setup and tear down inside the network. It ispossible that different packets of the same connection may be sent overdifferent paths from the source to the destination.

Without state information inside the network, it is difficult for anetwork to provide end-to-end QoS guarantees. In order to provide theseguarantees, the current trend is to maintain per-session stateinformation or the aggregation of this state information in the network.State information can be maintained in two ways. Soft state refers tostate that is periodically refreshed by the reservation protocoldeployed in the network, so, when lost, the state will be automaticallyreinstated. For example, in the RSVP protocol mentioned earlier, twokinds of soft state information are maintained: path state andreservation state. Each data source periodically sends a path messagethat establishes or updates the path state, while each receiverperiodically sends a reservation message that establishes or updates thereservation state. In contrast, hard state, such as the state maintainedin an ATM network, is established at connection setup time and ismaintained for the lifetime of the connection. Researchers havesuggested maintaining per-flow state only at the edge of a network,while the network core maintains only aggregated state information.

As is known to those of ordinary skill in the art, the next generationof the Internet Protocol (known as “IPv6”) provides enhancements overthe capabilities of the existing Internet Protocol version 4 (“IPv4”)service. Specifically, IPv6 adds capabilities to label packets belongingto particular traffic “flows.”A flow in IPv6 is a sequence of packetssent from a particular source to a particular destination. For example,the 24 bit Flow Label field in the IPv6 header may be used by a sourceto label those packets for which it requests special handling by theIPv6 routers, such as non-default quality of service or “real-time”service. The nature of that special handling might be conveyed to therouters by a control protocol such as RSVP, or by information within thepackets comprising the flow (e.g., in a hop-by-hop option).

As used herein, the term “flow” refers to the concept of a virtualcircuit in ATM networks aid of a flow in IP networks. Thus, a “flow” canbe a hard-state virtual circuit as in an ATM network, a soft-state flowas defined in IPv6, or a stateless connection such as a TCP connectionin an IP network. A flow (also called a “session”) may use either asingle path or multiple paths. For traffic requiring resourceguarantees, it is assumed that the traffic source specifies its trafficcharacteristics and desired performance guarantees. The network performsadmission control to decide if the traffic should be admitted. Once aflow is admitted, resources along its path are reserved through areservation protocol, such as RSVP.

For the purposes of the present invention, there are two different typesof flows: guaranteed flows and best-effort flows. Guaranteed flowsrequire QoS guarantees in terms of delay, delay jitter, and bandwidth.In contrast, best-effort flows require no firm QoS guarantees but expectthe network to deliver packets to their destinations as quickly aspossible. For guaranteed flows, there are two different types of QoSrequirements: bandwidth guarantees and delay guarantees. For best-effortflows, there are two subclasses: high-bandwidth traffic and low latencytraffic.

Best-effort traffic has been and will continue to be the main trafficclass in data networks for the foreseeable future. However, with theincreasing data sizes used by applications and the increased networkcapacity, large data sets from a few megabytes to several gigabytes areoften transmitted over the network. In contrast to traditionalbest-effort traffic, such as electronic mail, telnet, and small filetransfers, this new class of applications can send data at a high burstrate and can make use of the high bandwidth available in the network.This new class of traffic is known as high-bandwidth traffic,traditional best-effort traffic is referred to as low-latency traffic.

Ideally, both high-bandwidth and low-latency traffic should betransmitted in the minimum possible time, but the factors that affectthe elapsed time are different for each class of traffic. A low-latencymessage may consist of only a few packets, and the packets should besent to their destination with the minimum per-packet end-to-end delayto achieve the minimum elapsed time. For high-bandwidth traffic,however, a message can consist of hundreds or thousands of packets. Whensending these packets to their destinations, the available bandwidth onthe path will dominate the elapsed time. Given the different trafficcharacteristics and the different dominating factors determining theperformance of low-latency traffic and high-bandwidth traffic, thenetwork should provide explicit support to optimize user satisfaction aswell as network resource utilization efficiency for these two classes oftraffic.

Among guaranteed traffic flows, a distinction may be made between thosethat require only bandwidth guarantees and those that also require delayguarantees. For a flow requiring bandwidth guarantees, the requestedbandwidth may either be the mini bandwidth or the average bandwidth.However as is known to those of ordinary skill in the art, for a flowrequiring delay guarantees, the QoS requirements also include delay,delay jitter, and loss. To provide such QoS guarantees, the networkneeds to reserve a certain amount of bandwidth at each node on the pathfrom the source to the destination. In many cases, much more bandwidththan the average traffic rate may have to be reserved if the requesteddelay bound is tight and the traffic source is bursty. In this case, thereserved but unused bandwidth may be used opportunistically by otherflows in the network. Ensuring that the link share for each flowcorresponds to its requested QoS is the responsibility of specializedscheduling algorithms known to those of ordinary skill in the art, whichare not discussed herein.

Having described various contexts in which aspects of the presentinvention may be implemented, one simplified embodiment of the inventionis now described. FIG. 7 is a block diagram illustrating an exemplarynetwork 700 in which an embodiment of the present invention has beenimplemented. As shown in FIG. 7, exemplary network 700 comprises eightrouters R1-R8 interconnected in a specific manner. Naturally, neitherthe number of routers nor the manner in which they are interconnected iscritical. It should also be noted that the routers shown in exemplarynetwork 700 may or may not all be within the same domain, as that termwas used earlier in this document. The only requirement is that network700 should be defined in such a way that it is possible to determineroutes through the network, given a source and destination pair. Foursources S1-S4 and two destinations D1-D2 are connected to exemplarynetwork 700, as shown in the figure. Sources S1-S4 and destinationsD1-D2 may be client or server computers, other routers, or any otherkind of network device identifiable by a unique address.

For the sake of simplicity, data flows through the network are shown asunidirectional arrows. In the simplified example shown in FIG. 7, eachnetwork segment (i.e., each direct connection from one node to anothernode) is labeled with the binary number representing the router numberof the destination for each segment (e.g., the segments between S2 andR2, S3 and R2, S4 and R2 are all labeled as “0010,” while the segmentbetween R3 and R8 is labeled as “1000”). In a practical implementation,the network segments may be labeled with a number representing the IPaddress of the appropriate next hop. In general, each router can forwardan incoming packet to a variety of destinations, although only R2 and R4are shown as having multiple output ports in the simplified diagramshown in FIG. 7.

In FIG. 7, R1 and R2 are “ingress routers” in the sense that they arethe only network nodes into which packets may arrive from outsidenetwork 700. Router R1 receives data packets from source S1, whilerouter R2 receives data packet from sources S2-S4. In the context of thepresent invention, all other routers in the network (i.e., R3-R8) areconsidered to be “core routers.” Packets exit the network at routers R7and R8, with destination D1 being accessible from both R7 and R8, butdestination D2 only being accessible from R7. The network segmentsterminating in D1 and D2 are not given the usual labels, because theyare directly connected to one or more network routers (i.e., R7 cantypically determine that it is directly connected to D1 and D2, and R8can typically determine that it is directly connected to D1).

As is well known to those of ordinary skill in the art, there are manydifferent possible routes from each source S1-S4 to each destinationD1-D2. Determining the optimal route from each source to eachdestination, based on various factors known to those of ordinary skillin the art, is the quintessential function performed by routingalgorithms, such as the various routing algorithms that have beendescribed in this document. The specific type or types of routingalgorithms used in network 700 are not important in the context of thepresent invention. The only requirement is that some sort of routingalgorithm is implemented, which can be used to determine the route fromeach source to each destination. Moreover, as mentioned earlier, theseroutes can be dynamically recalculated based on various factors known tothose of ordinary skill in the art.

As an example, assume that, at any given point in time, whatever routingalgorithms are being used in network 700 shown in FIG. 7 determine thatthe following explicit routes should be used for the following sourceand destination pairs: SOURCE/DESTINATION PAIR EXPLICIT ROUTE <S1, D1>R1, R3, R8 <S2, D1> R2, R3, R8 <S3, D1> R2, R4, R3, R8 <S4, D1> R2, R1,R3, R8 <S2, D2> R2, R4, R6, R7

According to aspects of the present invention, each of the aboveexplicit routes can be assigned a “global path identifier” (or “globalpath ID”) by performing a bit-wise Exclusive-Or (“XOR”) function of allthe labels of the network segments along the route. Thus, the globalpath ID for <S1, D1> would be calculated by calculating the bit-wise XORfunction of the network segments along the route defined by “R1, R3,R8,” as follows:Global path ID<S1, D1>=0001XOR0011XOR1000=1010

Similarly, the global path ID's for the remaining routes defined aboveare calculated as follows:Global path ID<S2, D1>=0010XOR0011XOR1000=1001Global path ID<S3, D1>=0010XOR0100XOR0011XOR1000=1101Global path ID<S4, D1>=0010XOR0001XOR0011XOR1000=1000Global path ID<S2, D2>=0010XOR0100XOR0110XOR0111=0111

Thus, even from this simplified example, it can be seen that uniqueglobal path ID's (with a high degree of probability) can be assigned toeach route. As mentioned earlier, a practical implementation would usethe IP address of each appropriate hop along the route instead of thesimple 4 bit binary numbers used in the above example. As those ofordinary skill in the art will recognize, this approach can be used withbinary numbers having any arbitrary number of bits.

According to aspects of the present invention, each ingress routermaintains an Explicit Route Table (“ERT”) which, given asource/destination pair, provides an ordered list of the network devicesalong the corresponding explicit route and a global path ID that isassigned to that route. An exemplary ERT is shown in FIG. 8, containingthe values calculated above with reference to exemplary network 700shown in FIG. 7. As shown in FIG. 8, ERT 800 comprises asource/destination pair column 810, a route column 820, and a globalpath ID column 830. To conserve memory space, each ingress router wouldtypically contain only entries that pertain to each specific ingressrouter. Thus, for example, in the table shown in FIG. 8, only the <S1,D1> entry would appear in the ERT of router R1, while only the remainingentries would appear in the ERT of router R2.

According to aspects of the present invention, in one embodiment, eachcore router in the network maintains an Explicit Forwarding InformationBase (“EFIB”) instead of an ERT. FIG. 9 is block diagram illustratingthe structure of an exemplary EFIB. As shown in FIG. 9, EFIB 900comprises an incoming global path ID column 910, and outgoing globalpath ID column 920, and a next hop column 930. As will be described morefully below, given an incoming global path ID in a data packet, EFIB 900provides the corresponding global path ID and the next hop to which theincoming data packet should be routed. In one embodiment, the outgoingglobal path ID is calculated by performing a bit-wise XOR function ofthe incoming global path ID with the identifying number of the currentnode (e.g., the IP address of the appropriate port on the currentrouter, in a practical implementation). Depending on each particularimplementation, this calculation may be performed as part of the tableinitialization process (i.e., when each path object is propagateddownstream), or may be calculated “on the fly” as each packet arrives.In the case of “on the fly” calculation, the outgoing global path IDcolumn 920 of EFIB 900 shown in FIG. 9 is not necessary.

As those of ordinary skill in the art will recognize, due tomathematical properties of the XOR function, this calculation guaranteesthat whenever routes merge in the network, the global path ID'sassociated with the routes will also merge. In accordance with theseproperties, the outgoing global path ID can be easily obtained simply byperforming a bit-wise XOR function of the incoming global path ID withthe identifying number of the current node (e.g., the IP address of theappropriate port on the current router, in a practical implementation).

Thus, using the global path ID as defined above, once two routes merge,only a single entry in the EFIB is needed for all of the routes thatmerge at a given node. This is significant because the number of entriesrequired in the EFIB is thus greatly reduced, resulting in a scalablesolution. According to the present invention, the size of the EFIB isproportional to the number of different routes used, and is independentof number of flows in the network.

As an example, FIG. 9 illustrates the EFIB for router R1 shown in FIG.7. Using the same examples and defined explicit routes as above, thereare two explicit routes defined that pass through router R1: the routecorresponding to <S1, D1> (i.e., route R1, R3, R8), and the routecorresponding to <S4, D1> (i.e., route R2, R1, R3, R8). It should benoted that these two routes “merge” at router R1. It can easily be shownby those of ordinary skill in the art that the global path ID's forthese two routes also merge at R1 to the value of “1010,” that thecorresponding outgoing global path ID at the output of router R1 wouldbe “1011,” and that the next hop for both routes would therefore berouter R3. Therefore, the single entry shown in FIG. 9 illustrates the“merging” effect of the global path ID's, because this single entrycorresponds to all routes that merge at router R1.

Before packets can be forwarded according to aspects of the presentinvention, the ERT and EFIB tables must be set up. FIG. 10 is a flowchart illustrating the process of setting up an ERT at an ingress routeraccording to an embodiment of the present invention. At step 1000, anexplicit route is defined for a source/destination pair using any methodknown to those of ordinary skill in the art. The route selection can bemade on a per-flow basis or on a per-class basis. Moreover, a trafficflow or a traffic class can use multiple routes simultaneously. As hasalready been mentioned, the techniques according to the presentinvention are independent of path selection algorithms that are used toselect explicit routes. At step 1010, the global path ID for eachexplicit route defined is calculated by performing a bit-wise XORfunction of all of the network segments along the route, in the mannerdescribed earlier. At step 1020, an entry is written into the ERT of theingress router for each explicit route that enters the network at thatspecific ingress router (e.g., in the example shown in FIG. 7, onlyroutes from S1 to either D1 or D2 would be included in the ERT of routerR1).

At step 1030, for each explicit route in the ERT of an ingress router, apath object is transmitted downstream to the next hop along the explicitroute, using either the reliable forwarding mode or the unreliable mode.In the former case, the path objects are forwarded to the next hop usingreliable transmission (e.g., TCP), which is similar to link state updatetechniques used in the BGP protocol, as is known to those of ordinaryskill in the art. In one embodiment, path objects are maintained as softstate, and may be transmitted periodically from an ingress node such asan edge router in case table entries have been flushed out and lost forany reason.

As mentioned earlier, according to aspects of the present invention,core routers in the network maintain an. Explicit FIB (“EFIB”), which isindexed by an incoming global path ID. This EFIB may be implemented aseither a separate routing table (analogous to using a TIB in tagswitching approaches known to those of ordinary skill in the art), or asan extension of the existing FIB in typical routers.

FIG. 11 is a flow chart illustrating the process of setting up an EFIBat a core router according to one embodiment of the present invention.At step 1100, a path object is received from the previous hop along anexplicit route. At step 1100, a look up is performed on the EFIB of thereceiving router using the global path ID in the received path object asan index. At step 1120, a decision is made depending on whether an entryindexed by the global path ID in the received path object already existsor not. If an entry already exists, then the process ends at step 1125,at which point a determination is made that a new EFIB is not necessary,probably because the received path object relates to a path that mergesat the router receiving the path object.

At step 1130, if the EFIB entry does not already exist, then thecorresponding outgoing global path ID is calculated by performing abit-wise XOR function of the incoming global path ID with the ID of thereceiving router (e.g., the IP address of the corresponding port on thereceiving router), and the next hop is determined based on the explicitroute information contained in the received path object. At step 1140,the resulting EFIB entry information is written into the EFIB, and thepath object is forwarded to the next hop along the explicit route, ifnecessary, at step 1150.

FIG. 12 is a flow chart illustrating the packet forwarding process at aningress router according to an embodiment of the present invention. Atstep 1200, a packet is received at an ingress router. At step 1210, alook-up is performed on the ERT of the ingress router using the <source,destination> data pair contained in the received packet as an index. Atstep 1220, a decision is made depending on whether a match is found inthe ERT. At step 1230, if no match is found, then a global path D may beset up in the ERT in the manner described earlier with reference to FIG.10. Alternatively, the packet may be forwarded using default routes, ifavailable.

At step 1240, if a match is found in the ERT, then the appropriateglobal path ID corresponding to the <source, destination> pair isretrieved from the ERT, along with information identifying the next hopalong the appropriate explicit route. At step 1250, the global path IDis inserted into the packet as the outgoing global path ID, and at step1260, the packet is forwarded to the next hop along the explicit routebased on the next hop information retrieved from the ERT.

FIG. 13 is a flow chart illustrating the packet forwarding process at acore router according to an embodiment of the present invention. At step1300, a router along an explicit path receives a packet from theprevious hop along the path. At step 1310, the router determines whetherthe received packet contains a global path ID according to the presentinvention. If not (e.g., if the field where the global path ID isexpected is empty or invalid), at step 1320, the process shown in FIG.13 terminates, and the packet is forwarded using the default routesprovided by the underlying routing algorithms, such as OSPF or BGP.However, if at step 1310, it is determined that the packet contains aglobal path ID according to aspects of the present invention, at step1330 a look-up is performed on the EFIB of the receiving router usingthe global ID of the received packet as an index. Assuming there is amatch in the EFIB, at step 1340, the corresponding outgoing global pathID and next hop information (both of which should have been previouslywritten into the EFIB in the manner described earlier with reference toFIG. 11) are extracted from the EFIB. At step 1350, the global path IDin the packet is replaced with the new outgoing global path ID retrievedfrom the EFIB, and at step 1460, the packet is forwarded to the next hopusing the next hop information retrieved from the EFIB.

As those of ordinary skill in the art will recognize, a benefit of thepresent invention is that, unlike other routing techniques, explicitroute tear down is not required. To support rerouting, a new path objectis simply sent downstream to modify the entries in the EFIB, and a newglobal path ID is calculated. Packets using the new route are theninserted into the network with new global path ID.

As described herein, the present invention discloses a novel approach tosupport dynamic selection of explicit routes and dynamic rerouting inthe Internet. The proposed technique is simple and scalable. It cansupport both flow-based and class-based dynamic traffic engineering. Thetechnique is independent of existing routing protocols, but uses theroute selected by existing routing protocols as default routes. Hence,it can be used in both intra-domain (e.g., OSPF, RIP, and EIGRP) andinter-domain (e.g., BGP) routing. Dynamic rerouting typically implieshigh implementation complexity and operational overhead. The proposedtechnique makes rerouting simple and relatively inexpensive.

As those of ordinary skill in the art will recognize, the techniquesproposed in the present invention significantly reduce the complexity oftraffic engineering as currently proposed in the IETF. Compared withother approaches known to those of ordinary skill in the art, the newtechniques according to the present invention are independent of theunderlying routing protocol, scalable (i.e., they require a small EFIBsize and support flow aggregation), and dynamic (i.e., they can supportboth class-based and flow-based explicit routes).

The techniques described herein according to aspects of the presentinvention may be implemented in routers or in any device having aplurality of output interfaces that forwards incoming data to one ormore of these output interfaces. As is known to those of ordinary skillin the art, the program code which may be required to implement aspectsof the present invention may all be stored on a computer-readablemedium. Depending on each particular implementation, computer-readablemedia suitable for this purpose may include, without limitation, floppydiskettes, hard drives, network drives, RAM, ROM, EEPROM, nonvolatileRAM, or flash memory.

While embodiments and applications of this invention have been shown anddescribed, it would be apparent to those of ordinary skill in the arthaving the benefit of this disclosure that many more modifications thanmentioned above are possible without departing from the inventiveconcepts herein. The invention, therefore, is not to be restrictedexcept in the spirit of the appended claims.

1. A method for routing a data packet through an explicit path in a datacommunication network, the data packet having an incoming global pathidentifier corresponding to the explicit path, the data communicationnetwork comprising a plurality of network nodes, each of the pluralityof network nodes including a network device, the explicit pathcomprising a plurality of hops including a prior hop, a current hop, anda next hop, each of the plurality of hops being associated with one ofthe plurality of network nodes, the method comprising: receiving thedata packet from the prior hop at the current hop in the network device;performing a look-up function into a forwarding table using an indexbased on the incoming global path identifier to determine a forwardingtable entry related to the next hop to which the data packet should beforwarded; extracting an outgoing global path identifier for the datapacket from the forwarding table entry, wherein the outgoing global pathidentifier for the data packet has been precalculated as a function of(1) the incoming global path identifier and (2) an identifier of thenetwork device receiving the data packet at the current hop; replacingthe incoming global path identifier with the outgoing global pathidentifier in the data packet; and forwarding the data packet to thenext hop.
 2. The method as defined in claim 1, wherein there are twoexplicit paths that merge at a network node and share a single sameentry in the forwarding table in all downstream network nodes.
 3. Themethod as defined in claim 2, wherein the network device has assigned toit an Internet Protocol (“IP”) address as the identifier of the networkdevice and the outgoing global path identifier is calculated byperforming a bit-wise Exclusive-Or function on the incoming global pathidentifier and the IP address.
 4. The method as defined in claim 2,wherein the outgoing global path identifier is inserted into an optionalfield of an Internet Protocol (“IP”) packet header for the data packet.5. The method as defined in claim 4, wherein the network device hasassigned to it an IP address as the identifier of the network device andthe outgoing global path identifier is calculated by performing abit-wise Exclusive-Or function on the incoming global path identifierand the IP address.
 6. The method as defined in claim 1, wherein thenetwork device has assigned to it an Internet Protocol (“IP”) address asthe identifier of the network device and the outgoing global pathidentifier is calculated by performing a bit-wise Exclusive-Or functionon the incoming global path identifier and the IP address.
 7. The methodas defined in claim 1, wherein the outgoing global path identifier isinserted into an optional field of an Internet Protocol (“IP”) packetheader for the data packet.
 8. The method as defined in claim 7, whereinthe network device has assigned to it an IP address as the identifier ofthe network device and the outgoing global path identifier is calculatedby performing a bit-wise Exclusive-Or function on the incoming globalpath identifier and the IP address.
 9. An apparatus for routing a datapacket through an explicit path in a data communication network, thedata packet having an incoming global path identifier corresponding tothe explicit path, the data communication network comprising a pluralityof network nodes, each of the plurality of network nodes including anetwork device, the explicit path comprising a plurality of hopsincluding a prior hop, a current hop, and a next hop, each of theplurality of hops being associated with one of the plurality of networknodes, the apparatus comprising: means for receiving the data packetfrom the prior hop at the current hop in the network device; means forperforming a look-up function into a forwarding table using an indexbased on the incoming global path identifier to determine a forwardingtable entry related to the next hop to which the data packet should beforwarded; means for extracting an outgoing global path identifier forthe data packet from the forwarding table entry, wherein the outgoingglobal path identifier for the data packet has been pre-calculated as afunction of (1) the incoming global path identifier and (2) anidentifier of the network device receiving the data packet at thecurrent hop; means for replacing the incoming global path identifierwith the outgoing global path identifier in the data packet; and meansfor forwarding the data packet to the next hop.
 10. The apparatus asdefined in claim 9, wherein there are two explicit paths that merge at anetwork node and share a single same entry in the forwarding table inall downstream network nodes.
 11. The apparatus as defined in claim 10,wherein the network device has assigned to it an Internet Protocol(“IP”) address as the identifier of the network device and the outgoingglobal path identifier is calculated by performing a bit-wiseExclusive-Or function on the incoming global path identifier and the IPaddress.
 12. The apparatus as defined in claim 10, wherein the outgoingglobal path identifier is inserted into an optional field of an InternetProtocol (“IP”) packet header for the data packet.
 13. The apparatus asdefined in claim 12, wherein the network device has assigned to it an IPaddress as the identifier of the network device and the outgoing globalpath identifier is calculated by performing a bit-wise Exclusive-Orfunction on the incoming global path identifier and the IP address. 14.The apparatus as defined in claim 9, wherein the network device hasassigned to it an Internet Protocol (“IP”) address as the identifier ofthe network device and the outgoing global path identifier is calculatedby performing a bit-wise Exclusive-Or function on the incoming globalpath identifier and the IP address.
 15. The apparatus as defined inclaim 9, wherein the outgoing global path identifier is inserted into anoptional field of an Internet Protocol (“IP”) packet header for the datapacket.
 16. The apparatus as defined in claim 15, wherein the networkdevice has assigned to it an IP address as the identifier of the networkdevice and the outgoing global path identifier is calculated byperforming a bit-wise Exclusive-Or function on the incoming global pathidentifier and the IP address.
 17. An apparatus for routing a datapacket through an explicit path in a data communication network, thedata packet having an incoming global path identifier corresponding tothe explicit path, the data communication network comprising a pluralityof network nodes, each of the plurality of network nodes including anetwork device, the explicit path comprising a plurality of hopsincluding a prior hop, a current hop, and a next hop, each of theplurality of hops being associated with one of the plurality of networknodes, the apparatus comprising: an input interface for receiving thedata packet from the prior hop at the current hop in the network device;table search logic for performing a look-up function into a forwardingtable using an index based on the incoming global path identifier todetermine a forwarding table entry related to the next hop to which thedata packet should be forwarded; path identifier assignment logic whichextracts an outgoing global path identifier for the data packet from theforwarding table entry, wherein the outgoing global path identifier forthe data packet has been pre-calculated as a function of (1) theincoming global path identifier and (2) an identifier of the networkdevice receiving the data packet at the current hop; path identifierinsertion logic which replaces the incoming global path identifier withthe outgoing global path identifier in the data packet; and packetforwarding logic for forwarding the data packet to the next hop.
 18. Theapparatus as defined in claim 17, wherein there are two explicit pathsthat merge at a network node and share a single same entry in theforwarding table in all downstream network nodes.
 19. The apparatus asdefined in claim 18, wherein the network device has assigned to it anInternet Protocol (“IP”) address as the identifier of the network deviceand the outgoing global path identifier is calculated by performing abit-wise Exclusive-Or function on the incoming global path identifierand the IP address.
 20. The apparatus as defined in claim 18, whereinthe outgoing global path identifier is inserted into an optional fieldof an Internet Protocol (“IP”) packet header for the data packet. 21.The apparatus as defined in claim 20, wherein the network device hasassigned to it an IP address as the identifier of the network device andthe outgoing global path identifier is calculated by performing abit-wise Exclusive-Or function on the incoming global path identifierand the IP address.
 22. The apparatus as defined in claim 17, whereinthe network device has assigned to it an Internet Protocol (“IP”)address as the identifier of the network device and the outgoing globalpath identifier is calculated by performing a bit-wise Exclusive-Orfunction on the incoming global path identifier and the IP address. 23.The apparatus as defined in claim 17, wherein the outgoing global pathidentifier is inserted into an optional field of an Internet Protocol(“IP”) packet header for the data packet.
 24. The apparatus as definedin claim 23, wherein the network device has assigned to it an IP addressas the identifier of the network device and the outgoing global pathidentifier is calculated by performing a bit-wise Exclusive-Or functionon the incoming global path identifier and the IP address.
 25. A programstorage device readable by a machine, tangibly embodying a program ofinstructions executable by the machine to perform a method for routing adata packet through an explicit path in a data communication network,the data packet having an incoming global path identifier correspondingto the explicit path, the data communication network comprising aplurality of network nodes, each of the plurality of network nodesincluding a network device, the explicit path comprising a plurality ofhops including a prior hop, a current hop, and a next hop, each of theplurality of hops being associated with one of the plurality of networknodes, the method comprising: receiving the data packet from the priorhop at the current hop in the network device; performing a look-upfunction into a forwarding table using an index based on the incomingglobal path identifier to determine a forwarding table entry related tothe next hop to which the data packet should be forwarded; extracting anoutgoing global path identifier for the data packet from the forwardingtable entry, wherein the outgoing global path identifier for the datapacket has been pre-calculated as a function of (1) the incoming globalpath identifier and (2) an identifier of the network device receivingthe data packet at the current hop; replacing the incoming global pathidentifier with the outgoing global path identifier in the data packet;and forwarding the data packet to the next hop.
 26. A method for routinga data packet through at least one explicit path in a data communicationnetwork, the data packet having a source/destination data pair, the atleast one explicit path having specified a plurality of network nodesthat form a route from a source network node to a destination networknode, the data communication network comprising a plurality of networknodes, the method comprising: receiving the data packet having thesource/destination data pair; performing a look-up function into aforwarding table using an index based on the source/destination datapair to determine a forwarding table entry related to a next networknode along the route to which the data packet should be forwarded;extracting a global path identifier for the data packet from theforwarding table entry, wherein the global path identifier for the datapacket has been precalculated for the at least one explicit path throughthe data communication network; inserting the global path identifierinto the data packet; and forwarding the data packet to the next networknode.
 27. The method as defined in claim 26, wherein the global pathidentifier is inserted into an optional field of an Internet Protocol(“IP”) packet header for the data packet.
 28. The method as defined inclaim 27, wherein each of the plurality of network nodes has assigned toit an IP address and the global path identifier is calculated byperforming a bit-wise Exclusive-Or function on the IP addresses of theplurality of network nodes comprising the at least one explicit path.29. The method as defined in claim 26, wherein each of the plurality ofnetwork nodes has assigned to it an Internet Protocol (“IP”) address andthe global path identifier is calculated by performing a bit-wiseExclusive-Or function on the IP addresses of the plurality of networknodes comprising the at least one explicit path.
 30. An apparatus forrouting a data packet through at least one explicit path in a datacommunication network, the data packet having a source/destination datapair, the at least one explicit path having specified a plurality ofnetwork nodes that form a route from a source network node to adestination network node, the data communication network comprising aplurality of network nodes, the apparatus comprising: means forreceiving the data packet having the source/destination data pair; meansfor performing a look-up function into a forwarding table using an indexbased on the source/destination data pair to determine a forwardingtable entry related to a next network node along the route to which thedata packet should be forwarded; means for extracting a global pathidentifier for the data packet from the forwarding table entry, whereinthe global path identifier for the data packet has been precalculatedfor the at least one explicit path through the data communicationnetwork; means for inserting the global path identifier into the datapacket; and means for forwarding the data packet to the next networknode.
 31. The apparatus as defined in claim 30, wherein the global pathidentifier is inserted into an optional field of an Internet Protocol(“IP”) packet header for the data packet.
 32. The apparatus as definedin claim 31, wherein each of the plurality of network nodes has assignedto it an IP address and the global path identifier is calculated byperforming a bit-wise Exclusive-Or function on the IP addresses of theplurality of network nodes comprising the at least one explicit path.33. The apparatus as defined in claim 30, wherein each of the pluralityof network nodes has assigned to it an Internet Protocol (“IP”) addressand the global path identifier is calculated by performing a bit-wiseExclusive-Or function on the IP addresses of the plurality of networknodes comprising the at least one explicit path.
 34. An apparatus forrouting a data packet through at least one explicit path in a datacommunication network, the data packet having a source/destination datapair, the at least one explicit path having specified a plurality ofnetwork nodes that form a route from a source network node to adestination network node, the data communication network comprising aplurality of network nodes, the apparatus comprising: an input interfacefor receiving the data packet having the source/destination data pair;table search logic for performing a look-up function into a forwardingtable using an index based on the source/destination data pair todetermine a forwarding table entry related to a next network node alongthe route to which the data packet should be forwarded; global pathidentifier assignment logic which extracts a global path identifier forthe data packet from the forwarding table entry, wherein the global pathidentifier for the data packet has been pre-calculated for the at leastone explicit path through the data communication network; global pathidentifier insertion circuitry for inserting the global path identifierinto the data packet; and packet forwarding logic for forwarding thedata packet to the next network node.
 35. The apparatus as defined inclaim 34, wherein the global path identifier is inserted into anoptional field of an Internet Protocol (“IP”) packet header for the datapacket.
 36. The apparatus as defined in claim 35, wherein each of theplurality of network nodes has assigned to it an IP address and theglobal path identifier is calculated by performing a bit-wiseExclusive-Or function on the IP addresses of the plurality of networknodes comprising the at least one explicit path.
 37. The apparatus asdefined in claim 34, wherein each of the plurality of network nodes hasassigned to it an Internet Protocol (“IP”) address and the global pathidentifier is calculated by performing a bit-wise Exclusive-Or functionon the IP addresses of the plurality of network nodes comprising the atleast one explicit path.
 38. A program storage device readable by amachine, tangibly embodying a program of instructions executable by themachine to perform a method for routing a data packet through at leastone explicit path in a data communication network, the data packethaving a source/destination data pair, the at least one explicit pathhaving specified a plurality of network nodes that form a route from asource network node to a destination network node, the datacommunication network comprising a plurality of network nodes, themethod comprising: receiving the data packet having thesource/destination data pair; performing a look-up function into aforwarding table using an index based on the source/destination datapair to determine a forwarding table entry related to a next networknode along the route to which the data packet should be forwarded;extracting a global path identifier for the data packet from theforwarding table entry, wherein the global path identifier for the datapacket has been precalculated for the at least one explicit path throughthe data communication network; inserting the global path identifierinto the data packet; and forwarding the data packet to the next networknode.